Experimental investigation of Lord Kelvin’s isentropic thermoelastic cooling and heating expression for tensile bars in two engineering alloys

Solids change temperature when rapidly and elastically stressed. The effect proposed by W. Thomson, who was ennobled as Lord Kelvin, is adiabatic thermoelastic cooling or heating depending on the sign of the stress. A highly sensitive radiometrically calibrated, cooled infrared camera was employed to measure temperatures both decreasing and increasing. Temperature measurements made from the reversible, elastic part of the stress–strain curve during rapid uniaxial tensile loading and unloading were investigated. The quasi-isentropic cooling temperature from the stress loading curve is recovered by heating after the specimen fractures when the load is released. These measurements establish for the first-time isentropic temperature recovery. The materials tested are an AISI 4340 steel and an aluminium 2024 alloy. Measurements of the stress cooling are −0.65±0.05 K/GPa for steel and −1.8±0.3 K/GPa for aluminium alloy. The thermoelastic heating is −1.2±0.3 K/GPa for steel and −1.8±0.2 K/GPa for aluminium. These values are compared to Thomson's theoretical thermoelastic prediction; comparisons are also made to equations of state tables. The experimental values are below theoretical predictions. The quasi-isentropic, elastic part of the temperature change is fully recovered after extensive plastic deformation by the fracture stress release wave.